he main function of the autonomic nervous system is to assist the body in maintaining a constant internal environment (homeostasis). When internal stimuli signal that regulation of the body’s environment is required, the central nervous system (CNS) and its autonomic outflow issue commands that lead to compensatory actions. For example, a sudden increase in systemic blood pressure activates the baroreceptors, which in turn modify the activity of the autonomic nervous system so that the blood pressure is lowered toward its previous level (see
The autonomic nervous system has both sensory and motor divisions. The motor division is further divided into the sympathetic and parasympathetic divisions. Because much of the autonomic nervous system’s actions relate to control of the viscera, it is sometimes called the visceral nervous system.
In service of its homeostatic function, the autonomic nervous system mediates visceral reflexes (e.g., the gastrocolic reflex, where stomach distention triggers peristalsis in the intestines) and provides sensory information to the CNS for the perception of the state of our viscera, a percept known to anyone who has eaten too much at a meal. More generally, activation of autonomic receptors can evoke a variety of sensory experiences such as pain, hunger, thirst, nausea, and a sense of visceral distention; these perceptions can then lead to compensatory voluntary behaviors that assist in maintaining homeostasis.
In addition to its central role in homeostasis, the autonomic nervous system also participates in appropriate and coordinated responses to external stimuli that are required for the optimal functioning of the somatic nervous system in performing voluntary behaviors. For example, the autonomic nervous system helps regulate pupil size in response to different intensities of ambient light, thus helping the visual system to operate over a large range of light intensity.
In this chapter, the enteric nervous system is also considered part of the autonomic nervous system, although it is sometimes considered a separate entity (see also ). In addition, because the autonomic nervous system is under CNS control, the central components of the autonomic nervous system are discussed in this chapter. These central components include the hypothalamus and higher levels of the limbic system, which are associated with emotions (see ) and with many visceral types of behavior (e.g., feeding, drinking, thermoregulation, reproduction, defense, and aggression) that have survival value.
Organization of the Autonomic Nervous System
The sensory autonomic neurons are located in the dorsal root ganglia and in the cranial nerve ganglia. Like the other neurons of the dorsal root ganglia, they are pseudounipolar cells with a peripheral axonal branch extended to one of the viscera and a central branch that enters the CNS. With regard to autonomic motor output, both the sympathetic and parasympathetic nervous systems use a two-neuron motor pathway, which consists of a preganglionic neuron, whose cell body is located in the CNS, and a postganglionic neuron, whose cell body is located in one of the autonomic ganglia (
Figs. 11.1 11.2 ). The targets of this motor
CHAPTER 11 The Autonomic Nervous System and Its Central Control 227
Projections of Parasympathetic parasympathetic nervous system
Facial (VII) nerve, Glossopharyngeal (IX) nerve • Fig. 11.1 Schematic Illustration of the Sympathetic and Parasympathetic Pathways. Sympathetic pathways are shown in red and parasympathetic pathways in blue. Preganglionic neurons are shown in darker shades, and postganglionic neurons, in lighter shades.
pathway are smooth muscle, cardiac muscle, and glands. The enteric nervous system includes the neurons and nerve fibers in the myenteric and submucosal plexuses, which are located in the wall of the gastrointestinal tract.